Criminal gangs are tricking East Europeans into sham Scottish marriages with Asian men - selling the women for sex, labour and passports - in an expose that campaigners say reveals the extent of human slavery worldwide.

The cross-border story of how destitute women are lured from Romania and Slovakia with the promise of work in Western Europe, then sold in Scotland as slaves is told in a BBC television documentary to be aired on Wednesday.

Anti-slavery activists said the story was no surprise, with an estimated 46 million people living in some form of modern slavery around the globe.

"Sadly, the issue of sham marriage and sexual exploitation of vulnerable women and girls is not a new phenomenon," said Justine Currell, Executive Director at Unseen, an anti-slavery group.

"This story highlights the depths that these gangs will go to make a profit, regardless of the terrible consequences for the individual," she told the Thomson Reuters Foundation.

"We must put a stop to this horrendous practice by locking up those responsible and protecting those most at risk of exploitation."

HUMANS FOR SALEIn the documentary - "Humans for Sale" - Europe's police agency Europol said Scotland was being targeted by non-European nationals, often men from Pakistan and India, who need to marry a European national to stay in Britain.

The expose showed how the women are sold to the men by East European criminal gangs, and can end up facing sexual abuse and domestic servitude.

"After the marriage, they (the women) are kept under control by the traffickers and are exploited as domestic service by the husband, but also raped and sexually exploited by fellow nationals of the traffickers," said Angelika Molnar, head of the agency's trafficking unit.

British government figures estimate between 10,000 and 13,000 people are living as slaves in Britain alone.

In 2015, it passed the Modern Slavery Act, introducing life sentences for traffickers and forcing companies to disclose efforts to make their supply chains free from slavery.

Worldwide, the estimate leaps to 45.8 million, according to the Global Slavery Index, which is an annual study of slavery around the world. Women and girls are especially vulnerable, and the documentary found one Slovakian girl who had been trafficked to Glasgow on three separate occasions.

The programme looked into dozens of suspicious marriages in Scotland and analysed how many of the couples were divorced after five years – the minimum time an applicant must live in the country before applying for permanent residency.

Of 70 suspicious marriages that were registered in one Glasgow district, 40 percent ended in divorce just after five years, according to the investigation.

The BBC team also visited Roma communities in Slovakia where poverty and desperate living conditions make women particularly vulnerable to trafficking gangs.